1. Field of the Invention
This invention relates to a photo-scanning device, and particularly to a photo-scanning device of post-objective type with a curvature of field reduced.
2. Description of the Prior Art
In optical photo-scanners used for laser printers, laser displays, etc., there are known a post-objective type in which an optical deflector is arranged after a beam has been formed into a condenser light flux by a focusing lens and a pre-objective type in which a focusing lens is arranged after the deflector. The former is subjected to focusing action prior to deflection and therefore the focusing lens is required to resolve on-axis image formation alone, thus enabling to make the lens structure simple, but the focusing point is generally formed along a curved surface. On the other hand, in the pre-objective type, the beam is incident on the focusing lens after deflection, and therefore, a focusing lens needs to be designed as a wide angle lens sufficient to cover the deflection angle, thus complicating the structure of the lens but enabling the focusing point to be formed along more of a planar image surface. At present, this type is principally used.